Selective catalytic reduction (SCR) is commonly used to remove NOx (i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NOx is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NOx.
Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst positioned in the exhaust gas stream causes a reaction between NOx present in the exhaust gas and a NOx reducing agent (e.g., ammonia) to reduce/convert the NOx into nitrogen and water.
In many applications, such as SCR systems for vehicles, for example, the storage of ammonia in the form of a pressurized liquid in a vessel may be too hazardous and a storage method involving absorption in a solid may circumvent the safety hazard of anhydrous liquid ammonia. For example, metal ammine salts are ammonia absorbing materials, which can be used as solid storage media for ammonia, which in turn, for example, may be used as the reductant in SCR to reduce NOx emissions from internal combustion engines in vehicles, see e.g., U.S. Pat. No. 8,088,201 and WO 1999/01205. The ammonia can be released from the ammine salts through thermal desorption, e.g., by external heating of a storage container, see e.g., id. and U.S. Patent App. Pub. No. 2010/0086467. The ammonia is released from an either adsorptive or absorptive solid storage medium, among others Sr(NH3)8Cl2 or Ca(NH3)Cl2 in granular form, in a storage container and temporarily stored as a gas in a buffer volume. The amount of ammonia to be supplied to a reaction volume in the vehicle's exhaust system is dosed under the control of an electronic controller according to the current operating state of the engine.
The ammonia is consumed during driving, and as a result, the storage medium is depleted over time. Eventually, the ammonia in a canister is depleted and must be recharged or replaced. Replacing the canisters too early is obviously undesirable, as it can result in increased cost and vehicle down time, for example. Conversely, if the user waits too long to recharge or replace the canister, the SCR system may stop functioning correctly, causing undesirable NOx emissions. Accordingly, it is desirable to be able to determine the fill level of the storage container and to alert the user when the canister is nearing depletion.